1. Field of the Invention
The invention is generally related to radiation detection and more particularly to a semiconductor detector of ionizing electromagnetic radiation, neutrons, and energetic charged particles.
2. General Background
The present state of the art in semiconductor radiation detection is silicon diodes, high purity germanium (cooled by liquid nitrogen), and compound semiconductors such as cadmium zinc telluride (CZT), and mercuric iodide. Each of these materials has one or more drawbacks regarding its use. Silicon has a low atomic number and is therefore primarily useful for the detection of charged particles and atomic x rays emitted from low atomic number elements. Germanium has a higher atomic number but, because of its low band gap energy, must be cooled by liquid nitrogen in a bulky, expensive, and possibly dangerous cryogenic system to reduce thermally generated noise. Compound semiconductors such as CZT and mercuric iodide have sufficiently high band gap to be useful at or near room temperature. However, CZT has been plagued by production problems resulting in polycrystalline ingots with twins, inclusions, and grain boundary defects. These defects can never be completely removed and are a consequence of CZT being a solid solution rather than a true compound. The result is that spectroscopy-grade crystals must be mined from bulk material. Mercuric iodide suffers from low carrier mobility, short carrier lifetime, space charge polarization, and surface degradation. In addition, mercuric iodide is an extremely soft material that is easily damaged by the slight pressure of an electrical connection and by temperatures over sixty degrees Celsius.